1. Field of the Invention
This invention relates to the production of a specific dialkylnaphthalene and more particularly concerns the highly selective production of 2,6-methylethylnaphthalene by the transethylation of 2-methylnaphthalene.
2. Description of the Prior Art
2,6-Naphthalene dicarboxylic acid is a monomer that is known to be useful for the preparation of a variety of polymers. For example, poly(ethylene 2,6-naphthalate) which has better heat resistance and mechanical properties than polyethylene terephthalate and is useful in the manufacture of films and fibers is prepared from 2,6-naphthalene dicarboxylic acid and ethylene glycol.
2,6-Dialkylnaphthalenes are desirable feedstocks for oxidation to 2,6-naphthalene dicarboxylic acid. A known conventional process for producing 2,6-naphthalene dicarboxylic acid comprises the oxidation of a 2,6-dialkylnaphthalene with oxygen in the liquid phase in an acetic acid solvent at an elevated temperature and pressure and in the presence of a catalyst comprising cobalt, manganese and bromine components.
Dialkylnaphthalenes can be found in low concentrations in refinery streams as mixtures of some or all of the many possible dialkylnaphthalene isomers. However, separation of these isomers is very difficult and expensive. Consequently, methods for producing specific dialkylnaphthalenes or mixtures of two or three specific dimethylnaphthalenes in high purity and quality are highly desirable. Olah et al., "Alkylation of Naphthalene with Alkyl Halides," Journal of American Chemical Society, 98:7, pages 1839-1842 (Mar. 31, 1976) disclose that theretofor there was no clear understanding of directive effects and selectivities for the Friedel-Crafts alkylation of naphthalene.
Since then, Japanese Kokai Patent Application Publication No. 61-83137 Apr. 26, 1986) discloses a synthesis involving the transalkylation of naphthalene or 2-methylnaphthalene in the presence of an aluminum chloride catalyst at 0.degree.-35.degree. C. in the liquid phase to produce a 2,6-dialkylnaphthalene. Suitable alkylating agents are disclosed as including durene, diethylbenzene, triethylbenzene, triisopropylbenzene, isopropylxylene, and dibutylbenzene. The reported results indicate a relatively low degree of selectivity for the formation of specific dialkylnaphthalenes. Furthermore, it is specifically stated that the disclosed alkylation method must be performed at 0.degree.-35.degree. C., preferably room temperature, and that the higher the reaction temperature, the lower the selectivity for the formation of beta-alkyl substituted naphthalene and especially 2,6-dialkylnaphthalene. In addition, although this published patent application specifically mentions durene (1,2,4,5-tetramethylbenzene) as an example of an alkylation agent, it contains actual examples that illustrate only the use as alkylating agents in the method disclosed therein of polyalkylbenzenes where the alkyl groups are larger than methyl groups, and indicates as follows that polyalkylbenzenes with alkyl groups other than methyl groups afford benefits in the method disclosed therein: "Polyalkylbenzenes with ethyl, propyl, or butyl groups with high-carbon alkyl groups have high reaction rates . . . " Moreover, this published Japanese patent application states that, when the naphthalene is solid at the reaction temperature, a solvent such as a paraffin or cycloparaffin should be employed. This published Japanese patent application also discusses the use of halogenated alkyls in the alkylation of naphthalenes as a prior art method which did not produce a betaalkyl naphthalene with the desired selectivity.
Japanese Kokai Patent Application Publication No. 62-252733 (Nov. 4, 1987) discloses a process for the transethylation of biphenyl with an ethylbenzene to form monoethylbiphenyl and diethylbiphenyl in the presence of a Friedel-Crafts catalyst, such as aluminum chloride, at 70.degree.-150.degree. C. This published Japanese patent application discloses that a reaction temperature of less than 70.degree. C. delays the reaction rate. The ring positions of the ethyl substituents in the ethylated biphenyl products are not disclosed. Suitable ethylbenzenes are disclosed as including ethylbenzene, diethylbenzene, triethylbenzene, tetraethylbenzene, other ethyl-substituted benzenes, ethyltoluene, diethyltoluene and other ethyl-substituted toluenes. Polyethylbenzenes containing relatively small amounts of monoethylbenzene, triethylbenzene and tetraethylbenzene can also be used advantageously.
Shimada et al., "Ethylation and Transethylation of Naphthalene," Bulletin of the Chemical Society of Japan, Vol. 48 (II), pages 3306-3308 (November, 1975), disclose the transethylation of naphthalene by ethylbenzene or ethylxylenes to form monoethylnaphthalenes in the presence of an aluminum chloride catalyst at 20.degree.-30.degree. C. The rates of transethylation with ethylxylene isomers were reported to decrease in the order of 1,2-dimethyl-4-ethylbenzene.gtoreq., 1,3-dimethyl-4-ethylbenzene.gtoreq., 1,4-dimethyl-2-ethylbenzene .gtoreq.1,3-dimethyl-5-ethylbenzene.
Japanese Patent Application 26/336, published on Oct. 18, 1989, discloses a method for the preparation of ethyldiphenylethane or diethyldiphenylethane by the transethylation of diphenylethane with polyethylbenzene(s) in the presence of a Friedel Crafts catalyst at 0.degree.-150.degree. C. Preferred catalysts are aluminum chloride, aluminum bromide and boron trifluoride. Transethylation of 1,1-diphenylethane by this method produces either 1-phenyl-1-ethylphenylethane, 1-phenyl-1-diethylphenylethane or 1,1-bis(ethylphenyl)ethane. The ring positions of the ethyl substituents in the ethylated products are not disclosed.
Thus, until recently, no existing method was known for the highly selective production of 2,6-dialkylnaphthalene or of a mixture of 2,6- and 2,7-dialkylnaphthalenes by a transalkylation process. Then Hagen et al., U.S. Pat. No. 4,873,386, which issued on Oct. 10, 1989, disclosed a method for producing 2,6-diethylnaphthalene, which comprises: reacting in the liquid phase at least one of naphthalene or 2-ethylnaphthalene as the feed with at least one of 1,4-diethylbenzene, 1,2,4-triethylbenzene, at least one tetraethylbenzene or pentaethylbenzene as the ethylating agent per mole of the feed by weight, in the presence of a Lewis acid catalyst selected from the group consisting of aluminum chloride, aluminum bromide, tantalum pentachloride, antimony pentafluoride, and red oil, at a level of from about 0.01 to about 1 mole of the catalyst per mole of the feed (for red oil, based on the aluminum chloride content of the red oil) by weight and at a temperature in the range of from about -10.degree. C. to about 100.degree. C. In particular, Hagen et al., disclose that 1,2,3,4- and 1,2,3,5-tetraethylbenzenes, as well as 1,2,4,5-tetraethylbenzene, are useful ethylating agents, but that hexaethylbenzene is not. Hagen et al. further disclose that 2,6-diethylnaphthalene is formed at a higher selectivity and yield when 2-ethylnaphthalene is transethylated and that pentaethylbenzene and any tetraethylbenzene are the preferred ethylating agents.
Furthermore, it has been discovered that the oxidation of 2,6-dialkylnaphthalene proceeds with substantially less by-product formation when the alkyl groups are ethyl groups than when the alkyl groups are methyl groups, and thus that the crude 2,6-naphthalene dicarboxylic acid formed by the oxidation of 2,6-diethylnaphthalene can be purified to polymer grade purity more readily than can crude 2,6-naphthalene dicarboxylic acid formed by the oxidation of 2,6-dimethylnaphthalene. For this reason, the aforesaid transethylation method of Hagen et al. is especially desirable.
However, because of the relative unavailability of 2-ethylnaphthalene compared to the greater availability of 2-methylnaphthalene for use as the preferred feedstock for the aforesaid method of Hagen et al., and because of the benefit in efficiency in oxidizing the 2,6-dialkylnaphthalene of the lowest possible molecular weight to 2,6-naphthalene dicarboxylic acid, it is highly desirable to devise a method for the Friedel-Crafts transethylation of 2-methylnaphthalene to 2,6-methylethylnaphthalene. 2,6-methylethylnaphthalene is a compromise oxidation feedstock which would afford the benefits of a 2,6-dialkylnaphthalene both having the next to the lowest molecular weight of any dialkylnaphthalene and having one ethyl substituent for oxidation with substantially less by-product formation. Thus, it is highly desirable to provide a method for producing 2,6-methylethylnaphthalene by transethylation of a more relatively available feedstock than 2-ethylnaphthalene.